Unraveling the Mysteries of Quercetin: Antioxidant, Anti-inflammatory, and Anti-Diabetic Properties

April 2022

What is quercetin?

An organic compound always contains carbon (inorganic compounds do not contain carbon). Phenol is an organic aromatic (i.e., ring) compound composed of a benzene ring (6 carbons) with a hydroxyl group (-OH) substitution. The chemical formula of phenol is, therefore, HO-C6H5. Polyphenols are composed of more than one phenol. There are thousands of polyphenol groups. The primary polyphenol groups are: (1) flavonoids (~60% of all polyphenols, including quercetin), (2) phenolic acids (~30% of all polyphenols, including stilbenes and lignans), and (3) polyphenolic amides (including capsaicinoids found in chili peppers). Quercetin is a flavonoid (subclass: flavonol). Flavonoids are plant compounds with a flavone backbone: a three-ringed molecule with hydroxyl [OH] groups attached. Flavonols are either glycosides [have a glycosyl (sugar) group attached] or aglycones (no attached sugars). Quercetin is an aglycone. Quercetin glycoside has a glycosyl (sugar) group attached. Most dietary intake of quercetin is in the form of quercetin glycoside.

Quercetin is a phytochemical. Phytochemicals are compounds made by plants ("phyto" means "plant"). Quercetin is found in many foods, including apples, berries, grapes, onions, shallots, tomatoes, seeds, and nuts. Quercetin is also found in beverages such as red wine, black tea, and green tea. Growing conditions affect the concentration of quercetin in foods. For example, organically grown tomatoes have a significantly higher quercetin contact compared to conventionally grown tomatoes.


An antioxidant inhibits oxidation. Oxidation occurs when electrons are lost (reduction occurs when electrons are gained). When a molecule is missing an electron (when it is oxidized), it becomes a highly reactive unstable molecule called a “free radical.” Free radicals attack nearby molecules to steal an electron, damaging nearby structures. When food is metabolized, free radicals are formed. Oxidative stress is a state when free radicals outnumber antioxidants. Free radicals play a role in cancer (by damaging DNA), heart disease, stroke, and the general process of aging.

Quercetin is an antioxidant, inhibiting the production of free radicals and scavenging free radicals. This antioxidant effect can protect the brain, heart, and other vital tissues.


Quercetin has been found to lower blood glucose levels, help preserve pancreatic beta-cell integrity and function (the cells that make insulin), and decrease symptoms of diabetic neuropathy.

Aldose reductase is an enzyme that catalyzes the conversion of glucose to sorbitrol. This reaction plays an essential role in the formation of diabetic cataracts. Quercetin inhibits aldose reductase in the lens, thereby inhibiting sorbitol accumulation in the lens and delaying onset of diabetes-induced cataracts.

Anti-inflammatory/Anti-Allergy Effect

Quercetin inhibits inflammation-producing enzymes such as such as cyclooxygenase (COX) and lipoxygenase (LOX). Anti-inflammatory medications such as ibuprofen and naproxen also work by inhibiting COX. Quercetin has antinociceptive (pain-relieving) effects by inhibiting pronociceptive cytokine production and free radical generation that causes inflammatory pain. In patients with osteoarthritis, quercetin decreases pain symptoms and increases daily activities such as walking and going up and down stairs.

Quercetin inhibits histamine release from mast cells and basophils. White blood cells, such as eosinophils and neutrophils, promote inflammation. Inflammation can help control disease, but when the inflammation is greater than necessary, inflammation can cause troublesome symptoms and even tissue damage. Quercetin reduces eosinophil and neutrophil counts and infiltration in lung tissue and inhibits asthmatic reactions.

The Future

These are only a few of the many amazing and promising potential health benefits of quercetin. The data we have on quercetin is mostly based on studying its general pharmacokinetics and animal studies. More human clinical studies are needed to show how these basic and animal science discoveries specifically pertain to us.


Statements regarding dietary supplements have not been evaluated by the FDA and are not intended to diagnose, treat, cure, or prevent any disease or health condition.